Molded articles of cellular polyurethane elastomers are produced by known commercial processes from polyisocyanates, relatively high molecular weight polyhydroxyl compounds, water as chain lengthening and blowing agent and, optionally, additional chain lengthening agents. Cellular polyurethane elastomers differ from polyurethane foams by their substantially higher density (ca. 0.3 to 0.8 g/cm.sup.3) and their superior physical properties and the possibilities of commercial application arising therefrom. Exceptionally high quality cellular polyurethane elastomers such as, for example, a product produced from 1,5-naphthylene diisocyanate, a linear polyethyleneadipate diol (molecular weight ca. 2000) and water are used on a large commercial scale for, inter alia, buffers and shock absorbing materials.
One general field of application is found in the motor vehicle industry where such materials are used primarily for shock absorbent and impact resistant buffers and bumpers. In particular, in the construction of shock absorbing struts, it is advantageous to replace the auxiliary springs made of rubber by springs made of cellular polyurethane elastomers since these differ from solid elastic materials by their substantially higher deformability. Deformations of up to 80% are quite possible in practice, for example in auxiliary springs in motor vehicles.
In the shock absorbing strut construction of motor vehicles, consisting of shock absorbers, spiral springs and a buffer of cellular elastomer, the spring elements of cellular polyurethane elastomer are pushed over the piston rod of the shock absorber. In this arrangement, the risk of soiling of the buffers frequently arises since water spray and dust are liable to penetrate the shock absorbing buffers due to their predominantly open-celled pore structure. Apart from the fact that the spring characteristics may thereby be altered, especially at low temperatures, signs of premature corrosion and abrasion are found on the piston rod of the shock absorber due to the effect of water and dirt.
In U.S. Pat. No. 4,280,007, it has already been proposed to produce cellular polyurethanes having a density of from 0.45 to 0.8 g/cm.sup.3 from aromatic polyisocyanates, polyhydroxyl compounds having a molecular weight of from 400 to 6000, water and optionally glycols having a molecular weight of from 62 to 250, with the addition of from 0.1 to 0.8% by weight of aromatic diprimary diamines. The addition of aromatic diamines makes it possible for molded articles with considerably improved heat resistance to be obtained. Lower internal mold pressures are also used for their production. Such cellular polyurethanes, however, lack impermeability to water when subjected to bending stresses. In addition, it has been found that the process according to the above-identified reference ensures the production of highly uniform and, at the same time, very finely cellular structures. The product so produced is easily removed from the mold without risk of tearing or bursting. Finally, a uniform surface skin is easily formed.
It was thus an object of the present invention to provide shock absorbing spring elements which could be produced from the usual formulations and by means of the processing apparatus conventionally used in practice and which would not only have equal or even improved mechanical properties (e.g. spring characteristics) but would, in addition, be impermeable to water. If the products are to be suitable for use as shock absorbing and spring elements in the motor vehicle sector, it is essential that they should have a very low capacity for water absorption even when subjected to repeated alternating loads. At the same time, a very uniform hydrophobic cellular polyurethane elastomer (preferably one based on polyester urethanes) should be obtained by a reliable and simple process. The heat resistance and dynamic properties should also be improved. Furthermore, it was desired to find a process by which very uniform, fine cellular (micro cellular) polyurethane elastomers which are impermeable to water and have a density in the range of from 0.3 to 0.8 g/cm.sup.3 could be obtained reproducibly in simple stirred-mixing apparatus and could be easily manufactured in molds.
It was surprisingly found that the aims mentioned above could be achieved by adding small quantities of substantially linear polysiloxanes, preferably organofunctional derivatives, in the preparation of the cellular elastomers, preferably at the prepolymer stage, these polysiloxanes optionally being used in the form of NCO-terminated or OH-terminated silicone prepolymers. Moreover, such additives have the effect of reducing the coefficient of sliding friction of the cellular elastomers so that the abrasion of material on the piston rod is substantially reduced when the spring element is subjected to dynamic stress. The impermeable, very finely cellular polyurethane elastomers were found to be particularly suitable for their purpose if a small quantity of aromatic diamines had been added during polyurethane synthesis because the uniformity, heat resistance and dynamic properties of the product and the reproducibility of its preparation were thereby substantially improved and the polyurethane reaction mixture was easier to handle due to the lower mold pressure which could be employed. The product, having a smooth surface, could be easily removed from the mold.